As taught by M. J. O'Brien et al., Semiconductor International (May 1988) pages 51-52, certain tertiary-butyl esters, such as t-butyl naphthalene carboxylate are useful as dissolution inhibitors for deep-UV (250-300 novolak) novolak base photoresists. Mid-UV photoresists (300-350 nanometers) combining chemical amplification and dissolution inhibition also are made possible as a result of the use of t-butyl ester dissolution inhibitors as taught by M. J. O'Brien and J. V. Crivello, "Proceedings of SPIE" (1988) 920, 42.
J. L. Holcombe and T. Livinghouse, in the Journal of Organic Chemistry 51 (1986) pages 111-113, indicate that the synthesis of t-butyl esters can be achieved by effecting reaction between isobutylene and an appropriate carboxylic acid. However, the reaction is effected at high pressure. A procedure for synthesizing t-butyl esters employing elevated pressure and sulfuric acid is also outlined by L. F. Fieser and M. Fieser in "Reagents for Organic Synthesis", Wiley, New York, Vol. 1 (1967) page 522.
A high pressure synthesis is also used to prepare the corresponding ethyl- or isopropyl-esters of carboxylic acids taught by M. Gruffaz and O. Micaelli, Chemical Abstracts 92: 22064a. The ethyl and isopropyl carboxylic acid esters are prepared by utilizing a trifluoromethane sulfonic acid catalyst.
Addition of acetic acid to cyclic and strained bicyclic olefins catalyzed by trifluoromethane sulfonic acid is taught by R. M. G. Roberts, J. C. S. Perkin II (1976) pages 1183-1190.
Although tertiary butylesters, useful as dissolution inhibitors for novolak photoresists can be made by employing techniques requiring the use of sulfuric acid and isobutylene at elevated pressures, it would be desirable to synthesize such tertiary butyl esters by effecting reaction between an organic carboxylic acid, and isobutylene at atmospheric pressure for a period of about one hour or less.
The present invention is based on the discovery that a highly efficient method of adding isobutylene to carboxylic acid at atmospheric pressure is provided to produce a variety of tertiary butylesters by contacting the carboxylic acid with isobutylene at a temperature of about -7.degree. C. or less in the presence of an effective amount of trifluoromethane sulfonic acid.